Make max tip age an option instead of chainparam

After discussion in #7164 I think this is better.

Max tip age was introduced in #5987 to make it possible to run
testnet-in-a-box. But associating this behavior with the testnet chain
is wrong conceptually, as it is not needed in normal usage.
Should aim to make testnet test the software as-is.

Replace it with a (debug) option -maxtipage, which can be
specified only in the specific case.